E0710: A folder's Middleware is malformed

A folder's Middleware wraps every page, page method handler and endpoint in its folder and below, root-first. It is a package-level function of one of two forms: func Middleware(next http.Handler) http.Handler, or a constructor func Middleware(deps…) web.Middleware whose dependencies the container provides. Reported at the function.

Wrong

package api

func Middleware(next http.Handler) error { … }

Right

package api

func Middleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("X-API", "1")
		next.ServeHTTP(w, r)
	})
}